Fix provider usage detail object initialization in provider_chat_model

[slothsh]

Overview

When using an OpenAI-compatible model through OpenRouter, successful responses containing usage details such as prompt_tokens_details and completion_tokens_details caused the provider model to fail during response processing:

cannot use operator[] with a string argument with an array

This prevented OpenRouter tool-calling workflows from continuing after the model returned a valid response.

Solution

Explicitly initialize TokenUsage::details_ as a JSON object before storing provider-specific usage fields:

UsageMetadata usage;
usage.tokens_.details_ = nlohmann::json::object();